High strength thin steel sheet having high hydrogen embrittlement resisting property and high workability

Abstract

The present invention provides a high strength thin steel sheet that has high hydrogen embrittlement resisting property and high workability. The high strength thin steel sheet having high hydrogen embrittlement resisting property has a metallurgical structure after stretch forming process to elongate 3% comprises: (i) 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 9% or less (may be 0%) in total of ferrite and pearlite in terms of proportion of area to the entire structure, wherein the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher, or (ii) 1% or more residual austenite in terms of proportion of area to the entire structure; mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher; mean length of minor axes of the residual austenite grains is 1 μm or less; minimum distance between the residual austenite grains is 1 μm or less; and the steel has tensile strength of 1180 MPa or higher.

1 . A high strength thin steel sheet having high hydrogen embrittlement resisting property and high workability, which comprises: 
 C: higher than 0.25 up to 0.60%;    Si: 1.0 to 3.0%;    Mn: 1.0 to 3.5%;    P: 0.15% or less;    S: 0.02% or less; and    Al: 1.5% or less (higher than 0%) in terms of percentage by weight, with balance of iron and inevitable impurities;    wherein    the metal structure after stretch forming operation with elongation of 3% comprises:    residual austenite; 1% by area or more in proportion to the entire structure;    bainitic ferrite and martensite: 80% or more in total; and    ferrite and pearlite: 9% or less (may be 0%) in total, while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher, and    the steel has tensile strength of 1180 MPa or higher.    
   
   
       2 . The high strength thin steel sheet according to  claim 1 , wherein the metal structure after said stretch forming operation with elongation of 3% further satisfies the requirements that: 
 mean length of minor axes of said residual austenite grains is 1 μm or less; and    minimum distance between said residual austenite grains is 1 μm or less.    
   
   
       3 . The high strength thin steel sheet according to  claim 1 , wherein 0.5% or less (higher than 0%) by weight of Al is contained.  
   
   
       4 . The high strength thin steel sheet according to  claim 1 , wherein 
 0.003 to 0.5% of Cu and/or    0.003 to 1.0% of Ni    in terms of percentage by weight are further contained.    
   
   
       5 . The high strength thin steel sheet according to  claim 1 , wherein 
 0.003 to 1.0% of Ti and/or V    in terms of percentage by weight are further contained.    
   
   
       6 . The high strength thin steel sheet according to  claim 1 , wherein 
 1.0% or less (higher than 0%) of Mo and    0.1% or less (higher than 0%) of Nb    in terms of percentage by weight are further contained.    
   
   
       7 . The high strength thin steel sheet according to  claim 1 , wherein 0.0002 to 0.01% of B in terms of percentage by weight is further contained.  
   
   
       8 . The high strength thin steel sheet according to  claim 1 , wherein at least one element selected from the group consisting of: 
 0.0005 to 0.005% of Ca;    0.0005 to 0.01% of Mg; and    0.0005 to 0.01% of REM    in terms of percentage by weight is further contained.    
   
   
       9 . A high strength thin steel sheet having high hydrogen embrittlement resisting property and high workability, which comprises: 
 C: higher than 0.25 up to 0.60%;    Si: 1.0 to 3.0%;    Mn: 1.0 to 3.5%;    P: 0.15% or less;    S: 0.02% or less; and    Al: 1.5% or less (higher than 0%) in terms of percentage by weight,    with balance of iron and inevitable impurities, wherein the metal structure after stretch forming operation with elongation of 3% comprises:    residual austenite; 1% by area or more in proportion to the entire structure;    while the mean axis ratio (major axis/minor axis) of said residual austenite grains is 5 or higher;    mean length of minor axes of said residual austenite grains is 1 μm or less; and    minimum distance between said residual austenite grains is 1 μm or less; and    the steel has tensile strength of 1180 MPa or higher.    
   
   
       10 . The high strength thin steel sheet according to  claim 9 , wherein 0.5% or less (higher than 0%) by weight of Al is contained.  
   
   
       11 . The high strength thin steel sheet according to  claim 9 , wherein 
 0.003 to 0.5% of Cu and/or    0.003 to 1.0% of Ni    in terms of percentage by weight are further contained.    
   
   
       12 . The high strength thin steel sheet according to  claim 9 , wherein 
 0.003 to 1.0% of Ti and/or V    in terms of percentage by weight are further contained.    
   
   
       13 . The high strength thin steel sheet according to  claim 9 , wherein 
 1.0% or less (higher than 0%) of Mo and    0.1% or less (higher than 0%) of Nb    in terms of percentage by weight are further contained.    
   
   
       14 . The high strength thin steel sheet according to  claim 9 , wherein 0.0002 to 0.01% of B in terms of percentage by weight is further contained.  
   
   
       15 . The high strength thin steel sheet according to  claim 9 , wherein at least one element selected from the group consisting of: 
 0.0005 to 0.005% of Ca;    0.0005 to 0.01% of Mg; and    0.0005 to 0.01% of REM    in terms of percentage by weight is further contained.